In a film coater, an applicator applies a film of coating onto a roll surface and the roll surface in turn, transfers the coating onto the surface or side of a moving paper web. Generally, the film coater rolls can be paired to form a nip through which the web runs and the coating is transferred.
While various forms of applicators have been used with film coaters, one version uses a form of short dwell time coater or applicator (SDTA) to apply the coating onto the web. Generally, placement of these applicators is in the III and IV quadrants (with reference to the conventional four quadrant I, II, III and IV of a 360° circle) in order to obtain proper drainage of the coating overflow at the front gap of such type coaters.
Normally, in film coaters, the web generally runs downward through the nip formed between the pair of rolls. However, it has come to be recognized that there are disadvantages to such arrangement. One disadvantage is that all the equipment required is crowded into the lower III and IV quadrants. Also, as the roll surfaces pass by the nip and separate from the web, a film split phenomenon or action occurs, producing another disadvantage. During such film split action, small droplets or mist of coating are formed between the separating roll surface and web. Such small droplets or misting tend to fall back onto the coated web, moving downwardly as it leaves the coating nip, producing non-uniform coating defects on the just coated web.
The excess coating droplets or mist falling onto the downwardly moving web, could still produce coating imperfections even if subsequently doctored, as starting with uneven coating cannot always be overcome to form uniformly coated paper. Film split phenomenon (as the paper is locally wetter) also raises surface fibers which contributes to surface roughness on the coated web. Also, film split phenomenon causes a local uneven pattern on the coated sheet surface referred to as “orange peel pattern”. Further, the higher the operating speed, the greater the problem “film split” and “film split” droplets or misting become. Prior art film coaters have had operating speeds for these reasons, producing smooth paper at about 4500 ft./minute (1370 meters/minute) or less on the lighter grade (28 to 34 lbs.) papers produced.